陆地脊椎动物过去曾多次返回大海。现代海豹和鲸由于是哺乳动物，因而是胎生的，它们的性别由基因型决定。但曾经在海洋里生活的很多爬行动物如沧龙、鱼龙和蛇颈龙的情况是什么样的呢？性别也许是由环境决定的或基因型决定的，它们也许是胎生，也许是卵生。

根据复杂的系统发生分析，Organ等人提出，过去到海里去生活的爬行动物不仅是胎生的（这一点是从化石记录知道的），而且它们的性别也是由基因型决定的。这样，它们就不需要返回陆地去生育后代（羊膜卵在水中会死亡），它们也便能够发生形态转变，变成高度进化的鱼类形式。

原始出处：

Nature 461, 389-392 (17 September 2009) | doi:10.1038/nature08350

Genotypic sex determination enabled adaptive radiations of extinct marine reptiles

Chris L. Organ1, Daniel E. Janes1, Andrew Meade2 & Mark Pagel2

1 Museum of Comparative Zoology, Harvard University, 26 Oxford Street, Cambridge, Massachusetts 02138, USA

2 School of Biological Sciences, University of Reading, Reading RG6 6AJ, UK

Adaptive radiations often follow the evolution of key traits, such as the origin of the amniotic egg and the subsequent radiation of terrestrial vertebrates. The mechanism by which a species determines the sex of its offspring has been linked to critical ecological and life-history traits1, 2, 3 but not to major adaptive radiations, in part because sex-determining mechanisms do not fossilize. Here we establish a previously unknown coevolutionary relationship in 94 amniote species between sex-determining mechanism and whether a species bears live young or lays eggs. We use that relationship to predict the sex-determining mechanism in three independent lineages of extinct Mesozoic marine reptiles (mosasaurs, sauropterygians and ichthyosaurs), each of which is known from fossils to have evolved live birth4, 5, 6, 7. Our results indicate that each lineage evolved genotypic sex determination before acquiring live birth. This enabled their pelagic radiations, where the relatively stable temperatures of the open ocean constrain temperature-dependent sex determination in amniote species. Freed from the need to move and nest on land4, 5, 8, extreme physical adaptations to a pelagic lifestyle evolved in each group, such as the fluked tails, dorsal fins and wing-shaped limbs of ichthyosaurs. With the inclusion of ichthyosaurs, mosasaurs and sauropterygians, genotypic sex determination is present in all known fully pelagic amniote groups (sea snakes, sirenians and cetaceans), suggesting that this mode of sex determination and the subsequent evolution of live birth are key traits required for marine adaptive radiations in amniote lineages.